Fast evaluation of average critical area for ic

ABSTRACT

Method and apparatus for approximating the average critical area of a layout or layout region, involving summing, over all the object segments of interest, respective critical area contribution values that are dependent upon particular layout parameters of the objects, each of the contribution values being representative of a plurality of defect sizes, and being defined such that for each defect size in the plurality of defect sizes, and for a particular defect type, the contribution values collectively count all critical areas arising due to the object segments of interest only once.

BACKGROUND

1. Field of the Invention

The invention relates to integrated circuit fabrication, and moreparticularly to methods for determining the average critical area of aparticular layout.

2. Description of Related Art

As VLSI technology moves to deep submicron, manufacturability and yieldrelated issues become increasingly important. Yield loss can be causedby many factors. One important factor is random defect yield loss, whichis related to the yield loss caused by contamination particles. Thedesign related parameter required for modeling random defect yield issometimes called the critical area.

Critical Area measures a design's sensitivity to the random particledefects. Much work has been done on extracting and calculating thecritical area for a given design layout. The main approaches fall intothe two main categories: Shape Expansion based methods and Monte Carlomethods. Conventional shape expansion based methods generally attempt tocalculate the critical area contributed by each object and for eachparticular defect size of interest. For each defect size, the methodapproximates the geographic union of the critical area contributions ofall the objects. The result is then averaged over all the defect sizes,weighted by the defect size distribution.

One problem with the conventional shape expansion methods is thatcalculation of a geographic union can be extremely time consuming. Somemethods approximate the geographic union, but only at the expense ofaccuracy. Other methods do not even attempt to approximate thegeographic union, and simply add all the critical area contributionstogether. The latter variation creates significant inaccuracies becauseoverlapping regions are counted twice or more: once for each object thatincludes the region in its critical area contribution. Many conventionalshape expansion methods also suffer because they require a separatecritical area calculation for each defect size of interest. Because eachcritical area calculation is so expensive, the number of discrete defectsizes for which it is calculated is often reduced, thereby degradingaccuracy of the results. If accuracy is to be improved by increasing thenumber of discrete defect sizes at which the critical area calculationis made, then runtimes can easily become prohibitive. Another problemwith conventional shape expansion methods is that there is no explicitformula available for total critical area. Thus it cannot be used toevaluate critical area as part of cost function for layout optimization.

In Monte Carlo based methods, a generator generates random defects withtheir sizes following the given defect size distribution function. Sincethe Monte Carlo based methods do not need to limit themselves to anyspecific defect size, they do not suffer from accuracy degradation dueto insufficient numbers of defect sizes tested. But accurate estimationmay still require huge runtimes due to the need to test huge numbers ofrandomly generated defects.

Embodiments of the present invention can avoid the above problems andothers by deriving an explicit formula for a weighted average“pseudo-critical area” contributed by each object in the layout regionunder study. Preferably the weighted average pseudo-critical areasdepend only on parameters of the layout, all of which can be extractedduring a single sweep through the objects in the region. The weightedaverage pseudo-critical area preferably already accounts for all defectsizes of interest, so it is not specific to any individual defect size.It is therefore unnecessary to perform the calculation separately foreach of many defect sizes. The weighted average pseudo-critical areapreferably is defined also such that regions that might, underconventional definitions, be included in the critical area contributedby more than one object, are allocated to such objects in anon-overlapping manner. Calculating the geographic union of the weightedaverage pseudo-critical areas in such an embodiment therefore can be assimple as summing them. The final result for total weighted averagecritical area can be another explicit formula, as a function of layoutparameters only.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with respect to specific embodimentsthereof, and reference will be made to the drawings, in which:

FIGS. 1, 2, 5, 6 and 18 illustrate critical area regions arising due towires on a layout.

FIGS. 3, 4, 7, 8, 9, 11, 12, 14 and 17 are flowcharts illustratingmethods according to the invention for calculating and using short andopen average critical areas of a layout.

FIGS. 10 and 13 illustrate several objects in a layout layer.

FIG. 15 illustrates the positions of the wires in a layout before andafter layout optimization.

FIG. 16 is a simplified block diagram of a computer system suitable foruse with embodiments of the present invention.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles and features disclosedherein.

The critical area A_(c) is defined as the total area of the regions onthe layout where the contamination particles must fall to causefunctional failures (such as open and short circuit failures). Forpurposes of the present discussion, contamination particles areapproximated as discs, and the location of the centerpoint is consideredto be the location of the particle. Other conventions will produce thesame or similar results. FIGS. 1 and 2 illustrate conventional shapeexpansion methods for the extraction of the short and open criticalareas caused by the particles with radius x/2, or size x (shown asshaded areas). Once the critical areas for all the wires are computed,the geometric union of these areas gives the total critical areaA_(c)(x) at the defect size x. The “weighted average critical area” isthe average over a number or range of defect sizes of the critical area.The weighted average critical area A_(cr) is calculated as

$\begin{matrix}{{A_{cr} = {\int_{x_{\min}}^{x_{\max}}{{A_{c}(x)}{f(x)}{x}}}},} & (1)\end{matrix}$

where x_(min) and x_(max) are the minimal and maximal defect sizes in arange of defect sizes, and f(x) is the defect size distributionfunction.

As is well know, the arithmetic average over a certain number of valuesis the summation of these values divided by the total number of thevalues. Equation (1) is a weighted average value. This means that eachparticipating value's contribution to the sum is weighted with someweighting function. The weighted average value is the weighted sumdivided by the total contribution of all the participating values. Inequation (1), the weighting function is the defect size distributionfunction. If the weighting function for all participating values isequal, then the weighted average is the same as the arithmetic average,and this is considered herein to be a special case of the term “weightedaverage”. One basic property of a probability distribution function isthat the sum of all the distribution function values over all thepossible defect sizes is equal to one. Mathematically, ∫f(x)dx=1.Therefore the weighted average critical area A_(cr) in (1) is equal tothe weighted sum of the critical areas for every defect size A_(c)(x)with the defect size distribution function as the weighting function. Inthe following content of this document, for simplicity of discussion,the term “weighted average” is sometimes abbreviated merely as“average”.

Also as is well known, an average over a number or range of valuesdiffers from the sum of such values only by a scaling factor. That is,an average over such values is calculated by summing the (optionallyweighted) values and dividing the sum by a scaling factor given byeither the number of values or their range. The scaling factor may beapplied separately to the sum, or may be applied to the values (or theirweighting factors) prior to the sum. In many situations, however, thescaling factor is ignored. For example, if the average will be used onlyin relation to other averages that have the same scaling factor, it isunnecessary to divide any of the sums by the scaling factor; one valuewill remain smaller or larger than another of the values whether or notthey have all been divided by the scaling factor. As another example, ifthe scaling factor will become subsumed within a calibration factor thatis determined empirically, it is unnecessary to apply it explicitly. Itis sufficient instead to simply allow the empirical calibration toinclude the scaling factor's effects. It is common in these situations,therefore, for practitioners to use the terms “average” and “sum”interchangeably, even though mathematically speaking they are not thesame in an absolute sense. That is, one may speak of calculating anaverage value, but the equations given for doing so merely calculate thesum; or vice-versa. Consistently with that practice, the two terms,“average” and “sum”, are used interchangeably herein.

In order to calculate the average total critical area, the totalcritical area at each particular defect size, which is denoted asA_(c)(x) in equation (1), is needed. It can be seen that at eachparticular defect size, the conventional shape expansion methodtypically computes the geometric union of the critical areas for all thewires to obtain the total critical area. Due to the existence of theoverlaps between the critical areas of different wires, the totalcritical area is not a linear sum of each individual wire's criticalarea. Several algorithms (mostly based on scan-line or quad-treedata-structures) have been proposed for efficiently computing thegeometric union. However, none of these methods can handle the geometricunion analytically. Therefore there is no way to predict the totalcritical area at a different defect size. The whole extraction andcomputation procedure therefore has to be repeated for different defectsizes.

FIG. 17 illustrates the general concept for the derivation of anexplicit formula for average total critical area. Step 1710 is toformulate the pseudo-critical area contribution of any arbitrary layoutobject for a given defect size. Since the pseudo-critical area isdefined in such a way that the actual critical area regions arising dueto more than one object are allocated among the pseudo-critical areas ofthese objects and any region in the actual critical area is included inone and only one pseudo-critical area, the total critical area will be asummation of all the pseudo-critical area contributions of all objectsof interest. This is the operation performed in step 1720. Because step1720 performs only a summation, the result obtained from that step is anexplicit formula as well. Thus the integration in step 1730 can beperformed analytically resulting in another explicit formula at the endof step 1730 for average total critical area. Since summation andintegration are interchangeable, we can perform step 1730 immediatelyafter step 1710, then at the end perform step 1720. FIG. 3 shows theflow sequence after swapping steps 1720 and 1730 in FIG. 17. In FIG. 3,the step 310 performs step 1710 followed by step 1730 in FIG. 17.

FIG. 3 illustrates the basic overall method for the use of an embodimentof the invention. Two major steps are involved. First, the methoddetermines an “average pseudo-critical area” contribution function foran arbitrary object, a function that is already representative of allthe defect sizes of interest. (As used herein, a value is consideredherein to be “representative of” a plurality of underlying values if ittakes into account all of such underlying values.) Second, the methodthen sums this function over all the objects of interest.

In particular, in step 310, an analytical defect size-independentaverage pseudo-critical area contribution function is developed for anarbitrary layout object. This function is already representative of alldefect sizes of interest. It is based on, but is not the same as, theactual critical area contribution at each defect size. In particular, asset forth in more detail below, the average pseudo-critical areacontribution function is defined so that critical area regions arisingdue to more than one object are allocated among the objects in such away as to avoid overlaps. The average pseudo-critical area contributionfunction preferably does depend upon layout parameters of the objectsunder consideration, such as the spacing between objects, the width ofthe objects, and the portion of objects that are visible to neighboringobjects. Certain prior art methods assume that all objects are spacedequally and have equal widths, a simplification that does not accordwith actual practice and which can degrade the accuracy of the results.The average pseudo-critical area contribution function preferably doesnot depend upon any other parameters that vary as a function of defectsize or for different ones of the objects under consideration.

In step 312, with the explicit formulae for short and open criticalareas available, an embodiment need only to go through every objects inthe layout once to extract the above layout parameters. At the end ofextraction, the layout parameters are substituted into the formulae andthe average pseudo-critical area contributions of all objects ofinterest are simply summed. Because of the way the averagepseudo-critical area contribution function is defined for purposes ofstep 310, this simple summation is equivalent to a geographic union.Step 312 results in a critical area value arising from all the objectsof interest in the layout, already averaged over all the defect sizes ofinterest.

The above discussion is for calculating the total average critical area.An embodiment of the invention can be used to calculate the totalcritical area for a single defect size as well. If that is desired,there is not much difference in computational cost between the methodsdescribed herein and some traditional methods. But for situations thatrequire average critical area, the methods described herein can be muchmore efficient.

After the average critical area A_(cr) over the layout objects ofinterest is calculated in step 312, it may be used for a number ofdifferent purposes. It may be used for yield prediction, for example, orfor choosing among two or more implementations for a particular design.It can also be used in the physical verification step to check whetherthe critical area is within a given constraint, or incrementalevaluation of the changes in the average critical area due to layoutmodifications. The method described herein can also be used in a costfunction for global or detail routing. In step 314, as yet anotherexample, the average critical area value determined in step 312 is usedin a cost function for post-route yield optimization. Many other usesfor the average critical area value are known, or will be apparent tothe reader.

FIG. 4 is a flowchart of an overall method for determining an averagepseudo-critical area contribution function of an arbitrary layoutobject, which is already representative of all defect sizes of interest.As used herein, the term “average” refers to an average over the defectsizes of interest rather than an average over more than one object.Initially, in step 410, an analytical pseudo-critical area contributionfunction of an arbitrary object relative to an arbitrary neighbor isdescribed as a function of layout parameters and defect size. At thisstage the function is still dependent upon the defect size, however thatquantity might be expressed in the particular embodiment. In theembodiments described herein, as previously mentioned, defects areapproximated as discs, the location of the disc centerpoint isconsidered to be the location of the disc, and the disc diameter istaken as its defect size. Other embodiments can make different shape andsize approximations for the defects under consideration, such assquares, rectangles or more complicated shapes. Preferably but notnecessarily the defect “size” is varied by a single variable in theaverage pseudo-critical area contribution function.

In step 412, the average pseudo-critical area contribution function isintegrated over all defect sizes of interest with a probabilitydistribution function of the defect sizes. Any distribution function canbe used, but the present embodiment uses the well-known function ofequation (3):

$\begin{matrix}{{f(x)} = \left\{ \begin{matrix}{{\frac{x}{x_{0}^{2}}\mspace{14mu} {if}\mspace{14mu} 0} < x \leq x_{0}} \\{{{\frac{x_{0}^{2}}{x^{3}}\mspace{14mu} {if}\mspace{14mu} x_{0}} \leq x \leq x_{\max}},}\end{matrix} \right.} & (3)\end{matrix}$

where x₀ is the minimal spacing in the design rules.

The formula for total critical area as a function of defect size will bedifferent for different kinds of yield-affecting conditions. By way ofexample, formulas for two such conditions, defects creating a risk ofshort circuit conditions and defects creating a risk of open circuitconditions, will now be derived. For purposes of the followingdiscussion, the term “wire” is used herein to denote a region ofelectrically conductive material in a layer. The term “net” is used todenote a group of electrically conductive materials that always has sameelectrical signal in the circuit. In the embodiments described herein, a“wire” is broken into “objects” at corners, and “objects” can be furtherbroken into “segments” at equation applicability condition boundaries,discussed below. However, it will be appreciated that in anotherembodiment, “segments” can be defined differently and, if necessary, theequations can be adjusted accordingly.

Formula for “Short” Critical Area as a Function of Defect Size

If a pair of parallel wires i and j belonging to different nets haveportions visible to each other, as shown in FIG. 5, the critical areafor this pair of wires at a particular defect size x is simply

$\begin{matrix}{{A_{c}(x)} = \left\{ \begin{matrix}0 & {{{{if}\mspace{14mu} x} < s_{ij}},} \\{\left( {x - s_{ij}} \right)b_{ij}} & {{{if}\mspace{14mu} x} \geq {s_{ij}.}}\end{matrix} \right.} & (4)\end{matrix}$

Equation (4) is an effort to formulate the critical area analytically.There are at least two major problems with this formulation. One is thatif total critical area is computed as

$\begin{matrix}{{{A_{c}(x)} = {\sum\limits_{i}{\sum\limits_{{nbr}\mspace{14mu} j\mspace{14mu} {of}\mspace{14mu} i}{A_{ij}(x)}}}},} & (5)\end{matrix}$

where the term “nbr j of i” is read, “neighbor j of object i,” then thesame critical area A_(ij)(x) and A_(ji)(x) is going to be doublecounted. Another major problem is that when the defect size is large,the critical area arising due to wire pair (i,j) might overlap with thecritical area arising due to another wire pair (i,k), such as area Eshown in FIG. 6. That is, a particle might short the wire pair (i, k)and the wire pair (i, j) at the same time, if the particle is largeenough. Equation (4) does not handle the overlap regions at all. Thesummation of such critical areas would result in double counting ofthese overlap regions.

In order to solve this problem, the present embodiment describes thetotal critical area as a summation of “pseudo-critical” areas betweenevery pair of “visible” object segments. One object segment isconsidered herein to be “visible” to another in a particular directionif a straight line can be drawn in that direction so as to intersectboth segments without crossing another object that affects the criticalarea of the layout region. The “visibility length” b_(ij) of a pair ofobject segments is defined herein as the length of one object segmentwhich is “visible” to the other; the visibility length is sometimesreferred to herein as the “mutual visibility length”, since it should bethe same regardless of from which segment the view is taken (sob_(ij)=b_(ji)).

The pseudo-critical area Â_(ij)(x) is defined in such a way that nocritical area region on the layout, which arises due to any neighboringpair of objects, is included in the “pseudo-critical” area of more thanone object pair. Stated mathematically:

Â _(ij)(x)∪Â _(mn)(x)=φ, ∀(i,j)≠(m,n).  (7)

Note that the subscript (i,j) is order sensitive, i.e.,Â_(ij)(x)≠Â_(ji)(x). In order to satisfy equation (7), pseudo-criticalareas are defined in the present embodiment such that critical arearegions that arise because of a particular pair of objects i and j areassigned either to object pair (i,j), or to object pair (j,i), or areallocated partially to each in a non-overlapping manner. As used herein,the segment pair (i,j) is referred to herein as an “ordered” pair,meaning the pair (i,j) is to be considered a separate pair from the pair(j,i). Accordingly, an iteration over all “ordered” pairs (i,j) wouldcount pair (i,j) separately from pair (j,i).

FIG. 6 shows an object i and two neighboring objects j and k on oppositesides (right and left, respectively). As shown in FIG. 6, object 600 hasbeen split into three segments 610, 612 and 614. Segment 610 has onevisible neighbor j (in the rightward direction) and no visible neighborson the other side (in the leftward direction). Segment 612 has visibleneighbors j and k on both sides (in both the leftward and rightwarddirections). Segment 614 has no visible neighbors, and is ignored in thefollowing analysis for short critical area because, in a simplification,it has no possibility of being shorted with any other object in thefigure regardless of defect size.

FIG. 6 shows that for the segment 610, the critical area region A+B+Cfor a defect size x that is larger than twice the spacing s_(ij) betweenobject segments i and j. It can be seen that this region extends from aposition to the left of the leftmost segment i, to a position to theright of rightmost segment j. As shown in FIG. 6, the overall region hasbeen divided into a region A, which extends from the leftmost boundaryof the overall region to the center of segment i, a region B whichextends from the center of segment i to the center of segment j, and aregion C which extends from the center of segment j to the rightmostboundary of the overall region. For this situation, the presentembodiment allocates region A to object pair (ij) and region C to objectpair (j,i), and allocates region B half to object pair (ij) and half toobject pair (j,i). Therefore, in the present embodiment, for segment610, the pseudo-critical area is defined as

Â _(ij)(x)

A+B/2, Â _(ji)(x)

B/2+C.  (9)

Note that the sum of the two pseudo-critical areas does equal the actualcritical area:

Â _(ij)(x)+Â _(ji)(x)=A+B+C=A _(ij)(x),  (10)

and note further that the condition of eq. (7) holds:

Â _(ij)(x)∪Â _(ji)(x)=φ.  (11)

Each portion of the overall critical area region of the object segmentpair (ij) is included in one and only one pseudo-critical area region.

Through analyses such as that above with respect to FIG. 6, the presentembodiment analytically defines a pseudo-critical area function for anysegment of wire i that has a visible neighbor j in exactly one directionas:

$\begin{matrix}{{{\hat{A}}_{ij}(x)}\overset{\Delta}{=}\left\{ \begin{matrix}{0,} & {{{{if}\mspace{14mu} x} \leq s_{ij}},} \\{{\left( {x - s_{ij}} \right){b_{ij}/2}},} & {{{{if}\mspace{14mu} s_{ij}} < x \leq {{2s_{ij}} + {\min \left( {w_{i},w_{j}} \right)}}},} \\{\frac{\left( {s_{ij} + {\min \left( {w_{i},w_{j}} \right)}} \right)b_{ij}}{2 + {\left( {\frac{x}{2} - s_{ij} - \frac{\min \left( {w_{i},w_{j}} \right)}{2}} \right)b_{ij}}},} & {{{{{if}\mspace{14mu} 2s_{ij}} + {\min \left( {w_{i},w_{j}} \right)}} < x \leq D_{\max}},}\end{matrix} \right.} & (12)\end{matrix}$

where b_(ij) is the length of the overlap portion between objectsegments i and j, s_(ij) is the spacing between object segments i and j,w_(i) and w_(j) are the wire-widths for the two wires, and D_(max) isthe maximum random defect size of interest. The layout parameters takeninto account in Eq. (12) are b_(ij), s_(ij), w_(i) and w_(j).

For segment 612, which has visible neighbors on both sides, the criticalarea region includes subregions G and H to the right of segment i,defined as set forth above for subregions B and C. To the left ofsegment i the critical area region includes subregions E and F, whichare distinct but in conjunction are defined as set forth above forsubregion A. In addition, to the left of segment i, subregion D+E is thecritical area between wire i and wire k. Subregion E is an overlapregion that arises both because of the proximity between wire i and wirej and the proximity between wire i and wire k.

In the present embodiment, the pseudo-critical area for segments havingvisible neighbors on both sides has one component Â_(ij)(x) accountingfor subregions arising because of the proximity of one neighbor j, andanother component Â_(ik)(x) accounting for subregions arising because ofthe proximity of the other neighbor k. Â_(ij)(x) is defined similarly toits definition for segments having only one visible neighbor j:

Â _(ij)(x)

F+G/2,  (16)

where the region F is the region of the critical area A_(ij)(x) thatgoes away from wire j in the direction opposite wire i, but is notcovered by the k-side critical area A_(ik)(x). The region E, which isthe overlap region between the critical areas A_(ij)(x) and A_(ik)(x),is included as part of the pseudo-critical area A_(ik)(x):

Â _(ik)(x)

D/2+E/2.  (17)

Similarly, regions included in Â_(ik)(x) should not be allowed go to theother side of wire i (farther from wire k than wire i) because that parthas been covered by Â_(ij)(x). The region H is already totally includedas part of pseudo-critical area Â_(ji)(x). Following these steps, everyregion in the shaded area as shown in FIG. 6 is included in exactly onepseudo-critical area. The present embodiment thus analytically defines apseudo-critical area function for any segment of wire i that has visibleneighbors j and k on opposite sides as set forth in Equations (21),(22), (23).When 2s_(ij)+min(w_(i),w_(j))<2s_(ik)+min(w_(i),w_(k)),

$\begin{matrix}{{{\hat{A}}_{ij}(x)}\overset{\Delta}{=}\left\{ \begin{matrix}{0,} & {{{{if}\mspace{14mu} x} \leq s_{ij}};} \\{\frac{\left( {x - s_{ij}} \right)b_{ij}}{2},} & {{{{if}\mspace{14mu} s_{ij}} < x \leq {{2s_{ij}} + {\min \left( {w_{i},w_{j}} \right)}}};} \\{\begin{matrix}{\frac{\left( {s_{ij} + {\min \left( {w_{i},w_{j}} \right)}} \right)b_{ij}}{2} +} \\{\left( {\frac{x}{2} - s_{ij} - \frac{\min \left( {w_{i},w_{j}} \right)}{2}} \right)b_{ij}}\end{matrix},} & {{{if}\mspace{14mu} \begin{matrix}{{{2s_{ij}} + {\min \left( {w_{i},w_{j}} \right)}} <} \\{x \leq {s_{ij} + s_{ik} + w_{i}}}\end{matrix}};} \\{\begin{matrix}{\frac{\left( {s_{ij} + {\min \left( {w_{i},w_{j}} \right)}} \right)b_{ij}}{2} +} \\{\begin{bmatrix}{\left( {\frac{x}{2} - s_{ij} - \frac{\min \left( {w_{i},w_{j}} \right)}{2}} \right) -} \\\left( {x - s_{ij} - s_{ik} - w_{i}} \right)\end{bmatrix}b_{ij}}\end{matrix},} & {{{if}\mspace{14mu} \begin{matrix}{{s_{ij} + s_{ik} + w_{i}} <} \\{x \leq {{2s_{ik}} + {2w_{i}} -}} \\{\min \left( {w_{i},w_{j}} \right)}\end{matrix}};} \\{\frac{\left( {s_{ij} + {\min \left( {w_{i},w_{j}} \right)}} \right)b_{ij}}{2},} & {{if}\mspace{14mu} \begin{matrix}{{2s_{ik}} + {2w_{i}} -} \\{{\min \left( {w_{i},w_{j}} \right)} < x \leq {D\; \max}}\end{matrix}}\end{matrix} \right.} & (21)\end{matrix}$

When 2s_(ij)+min(w_(i),w_(j))>2s_(ik)+min(w_(i),w_(k)),

$\begin{matrix}{{{\hat{A}}_{ij}(x)}\overset{\Delta}{=}\left\{ \begin{matrix}{0,} & {{{{if}\mspace{14mu} x} \leq s_{ij}};} \\{\frac{\left( {x - s_{ij}} \right)b_{ij}}{2},} & {{{{if}\mspace{14mu} s_{ij}} < x \leq {{2s_{ij}} + {\min \left( {w_{i},w_{j}} \right)}}};} \\{\begin{matrix}{\frac{\left( {s_{ij} + {\min \left( {w_{i},w_{j}} \right)}} \right)b_{ij}}{2} +} \\{\left( {\frac{x}{2} - s_{ij} - \frac{\min \left( {w_{i},w_{j}} \right)}{2}} \right)b_{ij}}\end{matrix},} & {{{if}\mspace{11mu} \begin{matrix}{{{2s_{ij}} + {\min \left( {w_{i},w_{j}} \right)}} <} \\{x \leq {{2s_{ij}} + {2w_{i}} -}} \\{\min \left( {w_{i},w_{k}} \right)}\end{matrix}}\mspace{11mu};} \\\begin{matrix}{\frac{\left( {s_{ij} + {\min \left( {w_{i},w_{j}} \right)}} \right)b_{ij}}{2} +} \\{{\left( {w_{i} - \frac{\min \left( w_{i,j} \right)}{2} - \frac{\min \left( {w_{i},w_{k}} \right)}{2}} \right)b_{ij}},}\end{matrix} & {{if}\mspace{14mu} {\begin{matrix}{{2s_{ij}} + {2w_{i}} -} \\{{\min \left( {w_{i},w_{k}} \right)} < x \leq {D\; \max}}\end{matrix}.}}\end{matrix} \right.} & (22)\end{matrix}$

When 2s_(ij)+min(w_(i),w_(j))=2s_(ik)+min(w_(i),w_(k)),

$\begin{matrix}{{{\hat{A}}_{ij}(x)}\overset{\Delta}{=}\left\{ \begin{matrix}{0,} & {{{{if}\mspace{14mu} x} \leq s_{ij}};} \\{{\left( {x - s_{ij}} \right){b_{ij}/2}},} & {{{{if}\mspace{14mu} s_{ij}} < x \leq {{2s_{ij}} + {\min \left( {w_{i},w_{j}} \right)}}};} \\\begin{matrix}{\frac{\left( {s_{ij} + {\min \left( {w_{i},w_{j}} \right)}} \right)b_{ij}}{2} +} \\{{\left( {\frac{x}{2} - s_{ij} - \frac{\min \left( {w_{i},w_{j}} \right)}{2}} \right)b_{ij}},}\end{matrix} & {{{if}\mspace{14mu} \begin{matrix}{{{2s_{ij}} + {\min \left( {w_{i},w_{j}} \right)}} <} \\{x \leq {s_{ij} + s_{ik} + w_{i}}}\end{matrix}},} \\\begin{matrix}{\frac{\left( {s_{ij} + {\min \left( {w_{i},w_{j}} \right)}} \right)b_{ij}}{2} +} \\{{\begin{pmatrix}{\frac{s_{ij} + s_{ik} + w_{i}}{2} - s_{ij} -} \\\frac{\min \left( {w_{i},w_{j\;}} \right)}{2}\end{pmatrix}b_{ij}};}\end{matrix} & {{{{{if}\mspace{14mu} s_{ij}} + s_{ik} + w_{i}} < x \leq {D_{\max}.}}\mspace{11mu}}\end{matrix} \right.} & (23)\end{matrix}$

It will be appreciated that the above formulas for the pseudo-criticalarea as a function of defect size derives from certain conventions abouthow various critical area subregions, which arise due to more than onepair of neighboring objects, are to be allocated in a non-overlappingmanner to each pair. Two examples of allocation strategies are includedin the above derivation. For subregions of the critical area that arelocated between the wire pair (ij), half of the subregion is assigned tothe pseudo critical area of wire pair (i,j), the other half is assignedto the pseudo critical area of wire pair (j,i). More generally, afraction δ of the critical area may be allocated to one of the pairs(i,j), and the remaining fraction (1−δ) allocated to the other pair(j,i), 0<=δ<=1. In the above derivation, δ=½.

Similarly, for overlapping subregions that arise because the criticalarea due to wire pair (i,j) overlaps with the critical are due to adifferent wire pair (i,k), and these regions are located between thewires i and k, then half of them are assigned to the pseudo criticalarea of wire pair (i,k), half of them are assigned to the pseudocritical area of wire pair (k,i), and none is assigned to that of thewire pair (i,j). Again, more generally, a fraction δ of the criticalarea may be allocated to one of the pairs (ij), and the remainingfraction (1−δ) allocated to the other pair (i,k), 0<=δ<=1. Othervariations will be apparent. In general, δ can be a function of thedefect size x, but in the present embodiment δ is constant for alldefect sizes.

Now that an analytical formula has been derived for the pseudo-criticalarea contribution function of an arbitrary object i relative to anarbitrary neighbor j or neighbors j and k, as a function of layoutparameters and defect size, as previously mentioned, an analyticalformula for the average pseudo-critical area contribution, averaged overall defect sizes of interest, can now be derived by integrating thisfunction over all defect sizes of interest with a probabilitydistribution function of the defect sizes. Using the probabilitydistribution function of eq. (3), the following functions are thusderived.

For object segments having only one side visible neighbor, the averageshort pseudo-critical area can be written as

$\begin{matrix}{A_{S\; 1{ij}} = {x_{0}^{2}{{b_{ij}\left\lbrack {\frac{1}{4_{Sij}} + \frac{\;_{Sij}}{4D_{\max}^{2}} - \frac{1}{2D_{\max}}} \right\rbrack}.}}} & (24)\end{matrix}$

For object segments having visible neighbors in two opposite directions,the average short pseudo-critical area can be written as follows.

When 2s_(ij)ij +min(w_(i),w_(j))<2s_(ik)+min(w_(i),w_(k)),

$\begin{matrix}{A_{S\; 2{ij}} = {x_{0}^{2}{{b_{ij}\begin{bmatrix}{\frac{1}{4_{Sij}} - \frac{1}{2\left( {s_{ij} + s_{ik} + w_{i}} \right)} +} \\{\frac{1}{4\left( {{2s_{ik}} + {2w_{i}} - {\min \left( {w_{i},w_{j}} \right)}} \right)} -} \\\frac{s_{ij} + {\min \left( {w_{i},w_{j}} \right)}}{4D_{\max}^{2}}\end{bmatrix}}.}}} & (25)\end{matrix}$

When 2s_(ij)+min(w_(i),w_(j))>2s_(ik)+min(w_(i),w_(k)),

$\begin{matrix}{A_{S\; 3{ij}} = {x_{0}^{2}{{b_{ij}\begin{bmatrix}{\frac{1}{4_{Sij}} - \frac{1}{4\left( {{2s_{ij}} + {2w_{i}} - {\min \left( {w_{i},w_{j}} \right)}} \right)} -} \\\frac{s_{ij} + {2w_{i}} - {\min \left( {w_{i},w_{j}} \right)}}{4D_{\max}^{2}}\end{bmatrix}}.}}} & (26)\end{matrix}$

When 2s_(ij)+min(w_(i),w_(j))=2s_(ik)+min(w_(i),w_(k)),

$\begin{matrix}{A_{S\; 4{ij}} = {x_{0}^{2}{{b_{ij}\left\lbrack {\frac{1}{4_{Sij}} - \frac{1}{4\left( {s_{ij} + s_{ik} + w_{i}} \right)} - \frac{s_{ik} + w_{i}}{4D_{\max}^{2}}} \right\rbrack}.}}} & (27)\end{matrix}$

The above functions describe analytically the average pseudo-criticalarea for each kind of object segment pair that includes a segment ofobject i. They satisfy the condition set forth in eq. (7) for everydefect size x of interest, and therefore can simply be summed over allsuch object segments and objects in order to calculate the geometricunion of the actual short critical area, and thus the total averageshort critical area of the layout. That is,

$\begin{matrix}{{A_{crOpen} = {\sum\limits_{i}{\sum\limits_{j}A_{sij}}}},} & (28)\end{matrix}$

where A_(sij) is the short critical area contribution value A_(s1ij),A_(s2ij), A_(s3ij) or A_(s4ij), of object segment i as related to objectsegment j, and is given by equation (24), (25), (26) or (27) as the casemay be.

Formula for “Open” Critical Area as a Function of Defect Size

Theoretically, the basic derivation of the open critical area formulaeis a dual of the derivation for short critical area. For completeness,we describe the formulation for open critical area in detail as well.

As shown in FIG. 18, open critical area is formulated for object i. Thesegment 1810 has visible neighbors j and k on both sides, with lengthb_(ijk). The segment 1820 has only one side neighbor j, with lengthb_(ij). The segment 1830 has no visible neighbors on either side, withlength l_(i0). For an object segment that has no visible neighbors, suchas segment 1830 in FIG. 18, it is assumed that the defect must overlapthe entire width w_(i) in order to risk an open defect. Thus for thissegment, the pseudo-critical area contribution in the present embodimentis defined to be the same as the actual open critical area. It is afunction of defect size x, as shown in equation (29).

$\begin{matrix}{{{\hat{A}}_{i}(x)}\overset{\Delta}{=}\left\{ {\begin{matrix}0 & {{{if}\mspace{14mu} x} \leq w_{i}} \\{\left( {x - w_{i}} \right)l_{i\; 0}} & {{{if}\mspace{14mu} w_{i}} < x \leq D_{\max}}\end{matrix}.} \right.} & (29)\end{matrix}$

where I_(i0) is the length of the segment that has no visible neighborson either side, and D_(max) is the maximum defect size. Integratingequation (29) with the probability distribution function in equation (3)yields

$\begin{matrix}{A_{o\; 1i} = {x_{0}^{2}{{l_{i\; 0}\left\lbrack {\frac{1}{2w_{i}} - \frac{1}{D_{\max}} + \frac{w_{i}}{2D_{\max}^{2}}} \right\rbrack}.}}} & (30)\end{matrix}$

For a segment i that has exactly one side neighbor j, such as segment1820 in FIG. 18, the open critical area of object i might overlap withopen critical area of object j if the defect size is large. For smalldefect sizes, there is no overlap and the pseudo-critical areacontribution is the same as the actual open critical area. Itsformulation is the same as in equation (29). With increasing defectsize, the pseudo-critical area grows as well. When the actual opencritical area of object i begins to overlap the actual open criticalarea of object j, in order to avoid calculating the overlapping regionmore than once, the pseudo-critical area contribution for object i inthis situation is defined so as to stop growing on the side that hasvisible neighbor j. The other side of the pseudo-critical area continuesgrowing with increasing defect size. The formulation for pseudo-criticalarea of the segment of object i that has only one side visible neighborj is

$\begin{matrix}{{{\hat{A}}_{ij}(x)}\overset{\Delta}{=}\left\{ \begin{matrix}0 & {{{if}\mspace{14mu} x} \leq w_{i}} \\{\left( {x - w_{i}} \right)b_{ij}} & {{{if}\mspace{14mu} w_{i}} < x \leq {s_{ij} + w_{i} + w_{j}}} \\{\left( {\frac{x}{2} - w_{i} + \frac{s_{ij} + w_{i} + w_{j}}{2}} \right)b_{ij}} & {{{{{if}\mspace{14mu} s_{ij}} + w_{i} + w_{j}} < x \leq D_{\max}},}\end{matrix} \right.} & (31)\end{matrix}$

where b_(ij) is the length for the segment that has only one sidevisible neighbor j. Integrating equation (31) with the probabilitydistribution function of equation (3) yields

$\begin{matrix}{A_{o\; 1{ij}} = {x_{0}^{2}{{b_{io}\begin{bmatrix}{\frac{1}{2w_{i}} - \frac{1}{4\left( {s_{ij} + w_{i} + w_{j}} \right)} -} \\{\frac{1}{2D_{\max}} - \frac{s_{ij} - w_{i} + w_{j}}{4D_{\max}^{2}}}\end{bmatrix}}.}}} & (32)\end{matrix}$

For a segment i that has two neighbors j and k on opposite sides, suchas segment 1810 in FIG. 18, the definition for the pseudo-critical areais similar to that for the segment with only one side visible neighbor.The key point is that whether or not one side of the pseudo-criticalarea should stop growing with increasing defect size is determined bywhether or not the actual critical area of this segment begins tooverlap the actual critical area of the neighboring object on that side.The formulation for pseudo-critical area for a segment of object i thathas visible neighbors j and k on both sides is therefore given byequation (33). This formulation, without loss of generality, assumes thespacing s_(ij) between object i and its one side neighbor j is smallerthan the spacing s_(ik) between object i and it's the other sideneighbor k.

$\begin{matrix}{{{\hat{A}}_{ijk}(x)}\overset{\Delta}{=}\left\{ {\begin{matrix}0 & {{{if}\mspace{14mu} x} \leq w_{i}} \\{\left( {x - w_{i}} \right)b_{ijk}} & {{{if}\mspace{14mu} w_{i}} < x \leq {s_{ij} + w_{i} + w_{j}}} \\{\left( {\frac{x}{2} - w_{i} + \frac{s_{ij} + w_{i} + w_{j}}{2}} \right)b_{ijk}} & {{{{if}\mspace{14mu} s_{ij}} + w_{i} + w_{j}} < x \leq {s_{ik} + w_{i} + w_{k}}} \\{\begin{pmatrix}{\frac{s_{ij} + w_{i} + w_{j}}{2} +} \\{\frac{s_{ij} + w_{i} + w_{k}}{2} - w_{i}}\end{pmatrix}b_{ijk}} & {{{{if}\mspace{14mu} s_{ik}} + w_{i} + w_{k}} < x \leq D_{\max}}\end{matrix}.} \right.} & (33)\end{matrix}$

where b_(ijk) is the length of the segment that has visible neighbors onboth sides. Integrating equation (33) with the probability distributionfunction of equation (3) yields

$\begin{matrix}{A_{0\; 3{ijk}} = {x_{0}^{2}{b_{ijk}\begin{bmatrix}{\frac{1}{2w_{i}} - \frac{1}{4\left( {s_{ij} + w_{i} + w_{j}} \right)} -} \\{\frac{1}{4\left( {s_{ik} + w_{i} + w_{k}} \right)} - \frac{w_{j} + w_{k} + s_{ij} + s_{ik}}{4D_{\max}^{2}}}\end{bmatrix}}}} & (34)\end{matrix}$

As with equations (24), (25), (26) and (27) for the short defectanalysis, equations (30), (32) and (34) describe analytically theaverage open pseudo-critical area for each kind of object segment in anobject i. They satisfy the condition set forth in eq. (7) for everydefect size x of interest, and therefore can simply be summed over allsuch object segments and objects in order to calculate the geometricunion of the actual open critical area. That is,

$\begin{matrix}{{A_{crOpen} = {\sum\limits_{i}{\sum\limits_{j}A_{oij}}}},} & (35)\end{matrix}$

where A_(oij) is the open critical area contribution value A_(oli),A_(o2ij) or A_(o3ijk), from equations (30), (32) or (34) above,depending on whether the object segment i has visible neighbors on zero,one or two sides respectively.

In the case of the open defect analysis, the average pseudo-criticalarea equations can be combined and simplified to the single form ofequation (36):

$\begin{matrix}{{A_{crOpen} = {x_{0}^{2}{\sum\limits_{i}\left\lbrack {A_{oi} + {\sum\limits_{{nbr}\mspace{14mu} j}A_{oij}}} \right\rbrack}}},{where}} & (36) \\{{A_{oi} = {{- \frac{l_{i\; 1}}{2D_{\max}}} + \frac{w_{i}l_{i\; 1}}{4D_{\max}^{2}} - \frac{l_{i\; 0}}{D_{\max}} + \frac{w_{i}l_{i\; 0}}{2D_{\max}^{2}} + \frac{l_{i}}{2w_{i}}}},} & (37) \\{{A_{oij} = {{- \frac{b_{ij}}{4\left( {s_{ij} + w_{i} + w_{j}} \right)}} - \frac{\left( {w_{j} + s_{ij}} \right)b_{ij}}{4D_{\max}^{2}}}},} & (38)\end{matrix}$

“nbr j” refers to a visible neighboring object segment j,l_(i) is the total length of object i,l_(i0) is the total length of the segments of object i that have novisible neighboring segments,l_(i1) is the total length of the segments of object i that have exactlyone side visible neighboring segment,and b_(ij) and s_(ij) are the overlap. length and spacing, respectively,between object i and its visible neighboring segment j.

The average open pseudo-critical area contribution arising from a singleobject p is therefore

$\begin{matrix}{{A_{op} + {\sum\limits_{{nbr}\mspace{14mu} q}A_{opq}}},{where}} & (39) \\{{A_{op} = {{- \frac{l_{p\; 1}}{2D_{\max}}} + \frac{w_{p}l_{p\; 1}}{4D_{\max}^{2}} - \frac{l_{p\; 0}w_{i}}{D_{\max}} + \frac{w_{p}l_{p\; 0}}{2D_{\max}^{2}} + \frac{l_{p}}{2w_{p}}}},} & (40) \\{{A_{opq} = {{- \frac{b_{ij}}{4\left( {s_{pq} + w_{p} + w_{q}} \right)}} - \frac{\left( {w_{q} + s_{pq}} \right)b_{pq}}{4D_{\max}^{2}}}},} & (41)\end{matrix}$

“nbr q” refers to a visible neighboring object segment q,l_(p) is the total length of object p,l_(p0) is the total length of the segments of object p that have novisible neighboring segments,l_(p1) is the total length of the segments of object p that have exactlyone side visible neighboring segment,and b_(pq) and s_(pq) are the overlap length and spacing, respectively,between object p and its visible neighboring object q.

Summing Pseudo-Critical Area Contributions of All Objects of Interest

Returning to FIG. 3, now that an analytical function has been developedfor a defect size-independent average pseudo-critical area contributionof arbitrary layout objects (step 310), the actual critical area of thedesign can be approximated quickly and with high accuracy simply bysumming the average pseudo-critical area contributions of all objects ofinterest in the layout (step 312). FIG. 7 is an overall flow chartillustrating one of many possible methods for calculating the sum.

Referring to FIG. 7, in step 710, the routine loops through all objectsp of interest. If the critical area of the entire layout layer isdesired, then step 710 might loop through all objects in the layoutlayer. If the critical area of only a region of the layer is desired,then step 710 might loop through only the objects in the region. Notethat the present discussion uses the subscripts p and q to identify anobject and its neighbors, respectively, so that the subscripts i and jcan be available to identify object segments and their neighboringsegments, respectively.

In step 712, the routine calculates the average short critical areacontributed by object p, and adds it to a sum. Similarly, in step 714,the routine calculates the average open critical area contributed byobject p, and adds that value to another sum. The routine then loopsback to step 710 to evaluate the next object p. When all desired objectshave been evaluated, the routine exits (step 716) with two sums: oneapproximating the average short critical area and one approximating theaverage open critical area of the layer or region of interest.

In the flowchart of FIG. 7, as in all flowcharts herein, it will beappreciated that many of the steps can be combined, performed inparallel or performed in a different sequence without affecting thefunctions achieved. FIG. 8 is one such variation. Referring to FIG. 8,in step 810, as in step 710, the routine loops through all objects p ofinterest. In step 812, though, the routine combines steps 712 and 714into a single step for calculating both the short and open critical areacontributions of the current object p, and adding them to respectivesums. The remainder of the flowcharts herein follow the FIG. 8 versionof the algorithm.

FIG. 9 is a detail of step 812 in FIG. 8, for calculating average shortand open critical area contributions of a particular object p. In step910, the routine first obtains layout parameters for object p relativeto each visible neighboring object q. The layout parameters include, forexample, s_(pq), b_(pq), w_(p) and w_(q). The neighboring objectsinclude those that are left, right, up and down relative to object p.

Because the algorithm as implemented herein treats all four directionsalike, conventions will be useful for defining terms like the “width”and “length” of an object. As used herein, a layout layer is consideredherein to have two orthogonal dimensions, sometimes referred to hereinas the horizontal, or x, dimension and the vertical, or y, dimension.FIG. 10 illustrates a sample layout region having five objects 1010,1012, 1014, 1016 and 1018. Objects 1010, 1014, 1016 and 1018 areoriented vertically, in the sense that their size is greater in the ydimension than in the x dimension. Similarly, object 1012 is orientedhorizontally, in the sense that its size is greater in the x dimensionthan in the y dimension. In general, the “width” of an object refers tothe size of the object in the same dimension as the object's spacing toanother object. Thus when considering the pair of objects 1018 and 1016,the “width” of object 1016 is its size in the x dimension. Whenconsidering the pair of objects 1012 and 1016, the “width” of object1016 is its size in the y dimension.

The algorithm need not take into account the contributions of every wireor every pair of wires in a fabrication layer. For example, whereas theembodiments described herein for approximating the average shortcritical area takes into account all objects p in the layer regardlessof orientation, another embodiment might consider only objects p thatare oriented in a preferred direction, horizontal or vertical, of thecurrent layer. A wire extending perpendicular to the preferred directionmight be skipped during the iterations of such an embodiment. Such asimplification would reduce the accuracy of the resulting critical areacalculation, but nevertheless might be appropriate in certaincircumstances. As another example, the calculation may be limited toonly a particular region of the layer. On the other hand, otherembodiments might yield better accuracy than in the present embodiment,such as by including diagonally neighboring objects q rather than onlythose that are left, right, up or down relative to the current object p.An embodiment could handle diagonally neighboring objects by enlargingeach object slightly in all four directions before using the method asotherwise described herein.

In step 912, object p is segmented at equation applicability boundaries.More particularly, it will be recalled that for the short critical areacalculation, each of the equations (24), (25), (26) and (27) apply underdifferent topological conditions: equation (24) applies to segments ihaving only one visible neighboring segment, equation (25) applies tosegments i having two visible neighboring segments j and k in oppositedirections and for which2s_(ij)+min(w_(i),w_(j))<2s_(ik)+min(w_(i),w_(k)), equation (26) appliesto segments i having two visible neighboring segments j and k inopposite directions and for which2s_(ij)+min(w_(i),w_(j))>2s_(ik)+min(w_(i),w_(k)), and equation (27)applies to segments i having two visible neighboring segments j and k inopposite directions and for which2s_(ij)+min(w_(i),w_(j))=²s_(ik)+min(w_(i),w_(k)). Step 912 thereforesegments object p into one or more object segments i, each of which fitsthe conditions for exactly one of these equations for its entire length.Referring to FIG. 6, for example, object segment 600 is broken up intosegment 610, for which equation (24) applies along its entire length,and segment 612, for which equation (25) applies along its entirelength. Segment 614 does not contribute to the short critical areacalculation, but it is not ignored because it does contribute to theopen critical area contribution. It will be appreciated that differentassumptions and simplifications in the derivations in step 310 (FIG. 3)might yield different equation applicability conditions and thereforemight, in different embodiments, require different segmentation (or nosegmentation) of object p.

In step 914, the routine calculates the short and open critical areacontributions for each segment i in object p, using the respectiveapplicable equation (24), (25), (26) or (27) for the short critical areaand (36) for the open critical area, and accumulates them intorespective short and open critical area sums.

FIG. 11 is a flowchart detail of step 910, for obtaining layoutparameters for object p relative to each visible neighboring object q.In step 1110, the routine first determines a bounding box for object p.This might involve, for example, determining the minimum and maximum xand y locations of the object. In step 1112, the bounding box isextended by the maximum defect size of interest, D_(max), in all fourdirections. It is not necessary to consider neighboring objects beyondthat distance, since no defect of any size of interest can overlap boththe current object and the object outside the extended bounding boxsimultaneously to thereby create a short circuit. In step 1114, allobjects q that touch the extended bounding box are identified. Fourarrays are created and associated with object p. One array identifiesall the objects q in a rightward direction from object p; a second arrayidentifies all the objects q in a leftward direction from object p; athird array identifies all the objects q in an upward direction fromobject p; and a fourth array identifies all the objects q in a downwarddirection from object P.

In step 1116, the routine loops through all four directions left, right,up and down. In step 1118, the routine loops through all objects q thatare located in the current direction (left, right, up or down) from thecurrent object p. In step 1120, it is determined whether object q is“visible” to object p, as that term is defined elsewhere herein. If not,then control returns to the looping step 1118 to consider the nextobject q in the current direction.

If object q is visible to object p, then in step 1122, the routinecalculates and stores the layout parameters for object p relative toobject q. In the present embodiment, such layout parameters include thevisibility length b_(pq), the spacing s_(pq), the width of each objectw_(p), w_(q), the starting position (x or y, for vertically andhorizontally neighboring objects q, respectively) of the portion ofobject p from which object q is visible, and the ending position (x ory) of the portion of object p from which object q is visible. In step1124, the layout parameters extracted in step 1122 are added to an arrayfor object p for the current direction.

Control then returns to looping step 1118 to consider the next object qin the current direction from object p. If there are no more objects qin the current direction from object p, then control returns to loopingstep 1116 to consider the next direction. After all four directions havebeen considered, then in step 1126 the routine exits. At the end of step1126 (i.e. at the end of step 910), object p has associated therewithfour arrays: one for neighboring objects q in each of the fourdirections. Each entry in each array identifies for the object pair(p,q) the following layout parameters: b_(pq), s_(pq), w_(q), thestarting position (x or y) of the portion of object p from which objectq is visible, and the ending position (x or y) of the portion of objectp from which object q is visible. The width w_(p) is stored only oncefor each array, and will be the size of object p in the horizontaldimension for the left and right arrays, or the size of object p in thevertical dimension for the up and down arrays.

Returning to FIG. 9, after step 910, step 912 first segments the objectp at equation applicability condition boundaries, possibly creating asub-array of entries for each object pair (p,q). Step 914 thencalculates and accumulates both the short and open average critical areacontributions for each segment identified in the sub-arrays. In anembodiment, however, these steps are re-organized so that the averageshort and open critical area contributions are calculated separately,but the segmentation and calculation steps for each individual one ofthese defect types are combined together into a single method. FIG. 12is a flowchart illustrating a combined segmentation and calculationmethod for the average short critical area calculation. In the method ofFIG. 12, the average short critical area contributions of the variousobject segments are calculated promptly as each segment is identified.No sub-arrays need be formed unless required in the particularembodiment for other reasons.

FIG. 12 will be described with respect to an example vertical object p(1310) in the layout example of FIG. 13. In this layout example, twoobjects 1312 and 1314 are disposed leftward from object p, at distinctvertical positions along the length of object p. Three additionalobjects 1316, 1318 and 1320 are disposed rightward from object p, alsoat distinct vertical positions along the length of object p. Object p'sleft array therefore contains two entries whereas its right arraycontains three entries. In this example, no objects neighbor object p inthe upward or downward directions, so the up and down arrays for objectp are empty. Object 1320 is shown oriented horizontally in order toemphasize that the method of FIG. 9 makes no distinction based on theorientation of neighboring objects.

Referring to FIG. 12, in step 1210, the routine first loops through thefour directions, leftward, rightward, upward and downward. The firstdirection in the present illustration will be leftward. In step 1212,the routine loops through all entries q in the array for the currentdirection. The left array in the current illustration contains twoentries, one each for neighboring objects 1312 and 1314. In step 1214,the routine then traverses the length of object q in the dimensionperpendicular to the current direction. For object 1312, for example,step 1214 traverses object 1312 vertically.

As the current object q is traversed, in step 1216, the routine examinesthe layout parameters in the current array entry, as well as those inthe array for the direction opposite the current direction (the arrayfor the rightward direction, during this first direction iteration), inorder to determine which short pseudo-critical area equation (24), (25),(26) or (27) applies at the beginning of the current segment i. Forpurposes of this determination, the current position along the length ofthe current object p is the start of an object segment i, the currentposition along the length of the current object q is the start of anobject segment j, and the current position along the length of an object(if any) visible in the direction opposite the current direction is thestart of an object segment k. The starting position x_(ijstart) ory_(ijstart) is noted.

In step 1218, the routine again examines the layout parameters toidentify the ending position along the object q beyond which theequation determined in step 1216 no longer applies. If the equationapplicable to the current segment i is an equation applicable only wherethere is only one visible neighboring object segment, then the endingposition is the x or y position at which the current object p or q ends,or a new object begins in the direction opposite the current direction,whichever is first. If the equation applicable to the current segment iis an equation applicable only where there are visible neighboringobject segments on both sides, then the ending position is the x or yposition at which the current object p or q ends, or the current objectin the direction opposite the current direction ends, whichever isfirst. In any case, the ending position x_(ijend) or y_(ijend) is noted.

In step 1220 the parameter b_(ij) is now calculated as|x_(ijend)−x_(ijstart)| or |y_(ijend)−y_(ijstart)|, as the case may be.In step 1222, the parameters s_(ij) and if applicable, s_(ik) and w_(k)are obtained from the array entries. w_(i) is known from the width w_(p)of the current object p. All the parameters are now available tocalculate the average short pseudo-critical area contribution of objectsegment i in object p relative to segment j in object q using thecurrently applicable equation. This is done in step 1224, and thecontribution is accumulated into the overall average short critical areavalue being calculated. Control then returns to traversing step 1214 toidentify the next object segment i on object p relative to neighboringobject q, and to calculate and accumulate its short critical areacontribution.

If the traversal of step 1214 has reached the end of the current objectq, then control returns to looping step 1212 to consider the next entryq in the array for the current direction. If there are no more entriesin such array (i.e. after consideration of both objects 1312 and 1314 inthe example of FIG. 13), then control returns to step 1210 to considerthe array for the next direction relative to object p. In the example ofFIG. 13, this iteration will consider the critical area contributions ofeach segment i along the length of object p relative to each of theobjects 1316, 1318 and 1320 in the array for the rightward directionrelative to object p. Once the arrays for all four directions relativeto object p have been exhausted, the total average short pseudo-criticalarea contribution of object p will have been accumulated.

In order to further remove redundancy in visiting all four directions,FIG. 12 can be applied after the left and right directions are finishedfor step 1116 in FIG. 11. That is, after finishing steps 1118-1124 inFIG. 11 for both the left and right directions, steps 1212-1224 in FIG.12 are applied for these two directions. Steps 1118-1124 in FIG. 11 arethen applied for both the top and bottom directions, followed byperforming steps 1212-1224 in FIG. 12 for the top and bottom directions.

For the average open critical area contribution of object p, ifequations having distinct applicability conditions were derived in step310, then a routine similar to that of FIG. 12 could be used. In thepresent embodiment, though, all such equations were combined into asingle equation (36) above, which does not require segmentation. Thecalculation of the open pseudo-critical area contribution of object ptherefore requires no more than the extraction of the parameters w_(p),w_(q), l_(p), l_(p0), l_(p1), and b_(pq) and s_(pq) for each neighboringobject q. FIG. 14 is a flowchart illustrating one method for performingthe calculation.

In step 1410 of FIG. 14, the routine first loops through all entries ofneighboring object q in all four arrays associated with the currentobject p. In step 1412, A_(opq) is calculated according to equation (41)and accumulated. Control then returns to looping step 1410 to considerthe next neighboring object q.

After all the A_(opq)'s have been summed, then in step 1414, the routinebegins a new loop to traverse the length of object p. In step 1416,values for l_(p0) and l_(p1) are accumulated during this traversal. Theroutine then returns to step 1414 to continue the traversal. The valuefor l_(p) need not be accumulated during this traversal because it istrivially available from the starting and ending positions (x or y) ofobject p.

In step 1418, after l_(p), i_(p0) and l_(p1) are available, the routinecalculates A_(op) using equation (40) and adds it to A_(opq) calculatedin steps 1410 and 1412. In step 1420, the routine exits with the averageopen pseudo-critical area contribution due to object p.

As previously mentioned with respect to FIG. 8, after the average shortand open pseudo-critical area contributions of object p have beendetermined, they are added to respective sums to accumulate the totalaverage open critical area over all objects and defect sizes ofinterest.

The above-described embodiments make certain assumptions andsimplifications which may slightly degrade the accuracy of the overallcritical area calculation. For example, the above-described embodimentsassume that all objects of interest are rectangular, and allcontamination particles are discs. The embodiments also assume that ashort defect cannot exist between two objects if they do not overlap ineither the vertical or horizontal dimension, a simplification that maynot be valid where the two objects are offset in one dimension onlyslightly. Different equations or methods can be derived that do not makesuch simplifications, and those equations or methods may be more precisewhere the assumptions of the embodiments described herein are notcompletely valid. On the other hand, equations can also be derived thatmake additional simplifications that are not made in the above-describedembodiments, and the average critical area calculations from suchequations may in that sense be correspondingly less precise. Forexample, one embodiment might consider only the nearest visible neighborj, instead of all visible neighbors. Another embodiment might consideronly the neighbor(s) with the most significant impact, if that can bedetermined economically.

In general, the equations used herein need only determine the criticalarea approximately, to a level of precision that is required for theuser's purpose. As one example, the pseudo-critical area regions at eachdefect size of interest might be assigned to the individual objectsegments in only “substantially” (to desired level of precision)non-overlapping manner. While exactness would be desirable, it isusually unnecessary and even with the techniques described herein it mayrequire more computation time and effort than appropriate for the user'sparticular purpose. Nevertheless, the techniques described herein permitmuch more exact critical area computations with far less computationtime and effort than required by conventional methods. As used herein,exactness and precision are included in, and considered special casesof, estimation and approximation. That is, a formula that estimates orapproximates a value may actually determine the value exactly, and aformula that determines a value exactly is still considered to“approximate” or “estimate” the value.

Use of Short Pseudo-Critical Area Techniques for Guiding LayoutOptimization

As described above with respect to FIG. 3, after the average criticalarea A_(cr) over the layout objects of interest is calculated in step312, one of the purposes for which it can be used is as part of a costfunction to optimize the yield for a routed design. In the past, mostapproaches for post-routing yield optimization fell into either of twomain categories. In one category, the critical area at one particulardefect size was used for determining the new position of an object. (Inthis section, the term “object” is used generally. It need not extendall the way from one corner to another, nor need it be limited in lengthby equation applicability conditions.) This solution was inadequatebecause an optimization tuned for one specific defect size is notnecessarily optimal for all defect sizes. In fact, it can be shown thatthe optimal location for an object for minimizing the local criticalarea is different for different defect sizes, and quite often inopposing directions. In the second category, the object in question wasmoved incrementally and iteratively until no further improvement in theaverage critical area is observed. This solution was inadequate becauseprior art methods of critical area computation have been inaccurate orextremely expensive or both, as described above.

Having an explicit formulation for average critical area as derivedabove, permits direct calculation of the optimal location for a group ofone or more objects for minimizing the local critical area. This is animprovement over most previous approaches for incremental post-routelayout optimization. It is more accurate than the approach that computesthe optimal position based on only one defect size, and it can be farless expensive than the repeated move-evaluate approach. The approachdescribed herein avoids these move-evaluate iterations and hence is lesscomputationally expensive.

For simplicity of presenting the post-routing yield optimizationalgorithm, we use the scheme of moving one object at a time as anillustrative example. For that purpose, we derive an explicit formulafor the optimal location of an object that minimizes the local averageshort critical area. One embodiment can impose a requirement that themovement of objects not alter the predefined routing topology (i.e. thatthe optimization does not allow movement of an object so that itoverlaps or moves beyond any other object of interest), whereas anotherembodiment has no such requirement.

An object's optimal position depends on its position relative to all itsvisible neighbors. In the present embodiment, the critical area of thatneighborhood is formulated for the target object to be moved, not thepseudo-critical area. The main reason is that pseudo-critical area isformulated for the purpose of calculating the total actual criticalarea. In order to formulate the total critical area as summation of thepseudo-critical areas, the definition of the pseudo-critical area forone particular object i includes only part of the actual critical areafor object pair (ij); the remaining part is included in thepseudo-critical area for object j. For layout optimization, it is onlynecessary to formulate the critical area for the objects being moved andtheir visible neighbors. In the scheme being described here, only oneobject is moved at a time. Hence, only the evaluation of the criticalarea contributed by the object pairs between the target object with itsvisible neighbors is needed. There is no need to evaluate the criticalareas contributed by the target object's neighbors with their ownneighbors. We also carefully handle the possible overlaps among thecritical areas contributed by the different object pairs between thetarget object with its visible neighbors. The formulae are shown byequations (42) and (43) below.

Purely for the purpose of simplicity in an illustration of how a methodaccording to this technique works, we now apply some simplifyingassumptions in order to derive equations (42) and (43): D_(max) isassumed to be infinite, and we consider only the short critical area(not the open critical area). Other embodiments may not make these sameassumptions. The exact formulation can be derived following the similaridea but requires extra bookkeeping.

The critical area between the target object i and all its left sidevisible neighbors is formulated in (42). The critical area between thetarget object i and all its right side visible neighbors is formulatedin (43).

$\begin{matrix}{{A_{cr}(i)}_{left} = {{\sum\limits_{{left}\text{-}{nbr}\text{-}j}\frac{b_{ij}}{2s_{ij}}} - \frac{b_{ij}}{4\left( {{2s_{ij}} + w_{j}} \right)} - {\sum\limits_{\substack{{segments}\mspace{14mu} {of}\mspace{14mu} j \\ {with}\mspace{14mu} 2\text{-}{side}\mspace{14mu} {nbrs}}}{\frac{b_{ijk}}{4\left( {s_{ij} + s_{ik} + w_{i}} \right)}.}}}} & (42) \\{{A_{cr}(i)}_{right} = {{\sum\limits_{{right}\text{-}{nbr}\text{-}k}\frac{b_{ik}}{2s_{ik}}} - \frac{b_{ik}}{4\left( {{2s_{ik}} + w_{k}} \right)} - {\sum\limits_{\substack{{segments}\mspace{14mu} {of}\mspace{14mu} i \\ {with}\mspace{14mu} 2\text{-}{side}\mspace{14mu} {nbrs}}}{\frac{b_{ijk}}{4\left( {s_{ij} + s_{ik} + w_{i}} \right)}.}}}} & (43)\end{matrix}$

In these equations, b_(ij) is the total length of object i that isvisible to one of its left side visible neighbors. b_(ik) is the totallength of object i that is visible to one of its right side visibleneighbors, and b_(ijk) is the total length of the segment of object ithat has visible neighbors on both sides.

The optimal location for object i, therefore, given the abovesimplifications and assumptions, is the value s_(ij)* that minimizes thesum of the average critical area on the left and the average criticalarea on the right of the object. That is:

$\begin{matrix}{s_{ij}^{*} = {\min\limits_{s}{\left\lbrack {{A_{cr}(i)}_{left} + {A_{cr}(i)}_{right}} \right\rbrack.}}} & (44)\end{matrix}$

If equations (42) and (43) are further approximated by replacing thesummation with only the one neighbor that contributes the most criticalarea on that side (or with the nearest visible neighbor on that side),and by assuming the same wire width for all the objects, then we canderive the optimal position for object i explicitly:

$\begin{matrix}{s_{ij}^{*} = {\frac{1}{1 + \sqrt{\frac{b_{ik}}{b_{ij}}}}.}} & (45)\end{matrix}$

Now that an explicit formula has been derived for the optimal positionof an object i, the following describes the basic flow of a post-routingoptimization algorithm. Reference will be made to the example layoutregion shown in FIG. 15, which shows an object 1510 under consideration,and left- and right-hand objects 1512 and 1514, respectively. Theinitial horizontal position of object segment 1510 is shown as 1510 a,and the final position, as optimized by the method below, is shown as1510 b. Two other objects 1516 and 1518, nominally upward and downwardfrom the object segment 1510 under consideration, provide restrictionson the segment of object 1510 that can be moved.

The embodiment described herein focuses on a single layer at a time andso vias are not touched. Hence, the only movable objects on a layer arethe wires. Typically, in routing, each layer has a preferred directionof routing, either horizontal or vertical. Since the majority of thewires on a layer are in the preferred direction, the direction ofmovement is chosen to be perpendicular to the preferred direction onthat layer. Furthermore, only the wires along the preferred directionare candidates for moving in the present embodiment. For clarity ofexplanation, the present description assumes that the preferreddirection in the current layer under consideration is vertical. Thealgorithm then is as follows:

First, for each vertical object in the layer, the difference between thecritical areas on both sides is computed. This difference is referred toherein as the optimization potential.

Second, the algorithm identifies the object having the largestoptimization potential, and shifts it to the optimal location computedusing eq. (45).

Third, the optimization potentials of the visible neighbors of theshifted object are recomputed (as their visible neighbors have changed),and the second step is repeated until all the movable objects have beenprocessed once.

Note that once an object is shifted to the optimal location, it islocked and is not re-visited for the remainder of the iteration. Sincethe optimal location of an object segment as computed in Eq. (45)depends of the positions of the visible neighbors, it may help to runseveral iterations of the above algorithm to take these modificationsinto account.

In order to shift the object segment to an optimal position (the secondstep in the algorithm above) without violating any design rules, thefollowing two steps should be done efficiently:

-   -   Spacing-visible neighbors: Find all the objects that are visible        to the object being moved, modulo the spacing rules. We call        these the “spacing-visible” neighbors to differentiate from the        visible neighbors used in the critical area computation. The        spacing-visible neighbors of all the movable objects in the        layout are computed in one sweep through the layout. An        efficient variant of the algorithm outlined in J. Fang, et. al.,        “A new fast constraint graph generation algorithm for VLSI        layout compaction”, in IEEE International Symposium on Circuits        and Systems, 1991, incorporated by reference herein, is used for        the purpose.    -   Wire-pushing: Given the spacing-visible neighbors and the        optimal position, push the object segment to the specified        optimal location. As shown in FIG. 15, in the basic idea of wire        pushing, the original object segment is replaced by a set of        horizontal and vertical objects such that most of the original        object segment is now located at the calculated optimal        position, while obeying the spacing rules of the given layer.        The actual pushing algorithm is a modification of the well-known        skyline algorithm described in Udi Manber, “Introduction to        Algorithms, A Creative Approach”, Addison-Wesley Publishing Co.,        1989, at pp. 102-104. The entire Manber book is incorporated by        reference herein.

FIG. 16 is a simplified block diagram of a computer system 1610 on whichembodiments of the present invention can be executed. Computer system1610 typically includes at least one processor 1614 which communicateswith a number of peripheral devices via bus subsystem 1612. Theseperipheral devices may include a storage subsystem 1624, comprising amemory subsystem 1626 and a file storage subsystem 1628, user interfaceinput devices 1622, user interface output devices 1620, and a networkinterface subsystem 1616. The input and output devices allow userinteraction with computer system 1610. Network interface subsystem 1616provides an interface to outside networks, including an interface tocommunication network 1618, and is coupled via communication network1618 to corresponding interface devices in other computer systems.Communication network 1618 may comprise many interconnected computersystems and communication links. These communication links may bewireline links, optical links, wireless links, or any other mechanismsfor communication of information. While in one embodiment, communicationnetwork 1618 is the Internet, in other embodiments, communicationnetwork 1618 may be any suitable computer network.

User interface input devices 1622 may include a keyboard, pointingdevices such as a mouse, trackball, touchpad, or graphics tablet, ascanner, a touchscreen incorporated into the display, audio inputdevices such as voice recognition systems, microphones, and other typesof input devices. In general, use of the term “input device” is intendedto include all possible types of devices and ways to input informationinto computer system 1610 or onto computer network 1618.

User interface output devices 1620 may include a display subsystem, aprinter, a fax machine, or non-visual displays such as audio outputdevices. The display subsystem may include a cathode ray tube (CRT), aflat-panel device such as a liquid crystal display (LCD), a projectiondevice, or some other mechanism for creating a visible image. Thedisplay subsystem may also provide non-visual display such as via audiooutput devices. In general, use of the term “output device” is intendedto include all possible types of devices and ways to output informationfrom computer system 1610 to the user or to another machine or computersystem.

Storage subsystem 1624 stores the basic programming and data constructsthat provide the functionality of certain embodiments of the presentinvention. For example, the various modules implementing thefunctionality of certain embodiments of the invention may be stored instorage subsystem 1624. These software modules are generally executed byprocessor 1614.

Memory subsystem 1626 typically includes a number of memories includinga main random access memory (RAM) 1630 for storage of instructions anddata during program execution and a read only memory (ROM) 1632 in whichfixed instructions are stored. File storage subsystem 1628 providespersistent storage for program and data files, and may include a harddisk drive, a floppy disk drive along with associated removable media, aCD-ROM drive, an optical drive, or removable media cartridges. Thedatabases and modules implementing the functionality of certainembodiments of the invention may be stored by file storage subsystem1628.

Bus subsystem 1612 provides a mechanism for letting the variouscomponents and subsystems of computer system 1610 communicate with eachother as intended. Although bus subsystem 1612 is shown schematically asa single bus, alternative embodiments of the bus subsystem may usemultiple busses.

Computer system 1610 itself can be of varying types including a personalcomputer, a portable computer, a workstation, a computer terminal, anetwork computer, a television, a mainframe, or any other dataprocessing system or user device. Due to the ever-changing nature ofcomputers and networks, the description of computer system 1610 depictedin FIG. 16 is intended only as a specific example for purposes ofillustrating the preferred embodiments of the present invention. Manyother configurations of computer system 1610 are possible having more orless components than the computer system depicted in FIG. 16.

As used herein, a given value is “responsive” to a predecessor value ifthe predecessor value influenced the given value. If there is anintervening processing step, the given value can still be “responsive”to the predecessor value. If the intervening processing step combinesmore than one value, the output of the processing step is considered“responsive” to each of the value inputs. If the given value is the sameas the predecessor value, this is merely a degenerate case in which thegiven value is still considered to be “responsive” to the predecessorvalue. “Dependency” of a given value upon another value is definedsimilarly. Also, a second value is considered herein to be “independent”of a first value if the first value is not referenced in the calculationof the second value.

As used herein, the “identification” of an item of information does notnecessarily require the direct specification of that item ofinformation. Information can be “identified” in a field by simplyreferring to the actual information through one or more layers ofindirection, or by identifying one or more items of differentinformation which are together sufficient to determine the actual itemof information. In addition, the term “indicate” is used herein to meanthe same as “identify”.

The foregoing description of preferred embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in this art. Inparticular, and without limitation, any and all variations described,suggested or incorporated by reference in the Background section of thispatent application are specifically incorporated by reference into thedescription herein of embodiments of the invention. The embodimentsdescribed herein were chosen and described in order to best explain theprinciples of the invention and its practical application, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

1-27. (canceled)
 28. A method for approximating the average over aplurality of defect sizes of the short critical area contributed by aset of at least two object segments in an integrated circuit fabricationlayer, comprising the steps of: summing, over all the object segments iin the set, respective critical area contribution values that aredependent upon particular layout parameters of the objects in the set,each of the contribution values being representative of the plurality ofdefect sizes, and each of the contribution values being defined suchthat for each defect size x in the plurality of defect sizes, and foreach pair of mutually visible object segments i and j in the set, forwhich x exceeds the spacing between object segments i and j, only partof the short critical area between object segments i and j is includedin the contribution value of object segment i and the remainder of theshort critical area between object segments i and j is included in thecontribution value of object segment j; and approximating the averagecritical area as a function of the sum.
 29. A method according to claim28, wherein in the step of summing, the part of the short critical areabetween object segments i and j included in the contribution value ofobject segment i is a fraction δ_(x) of the short critical area betweenobject segments i and j, 0<=δ_(x)<=1, δ_(x) being constant for alldefect sizes x in the plurality of defect sizes.
 30. A method accordingto claim 29, wherein δ_(x)=0.5.
 31. A method according to claim 28,wherein the particular layout parameters of a given pair of objectsegments include the mutual visibility length of the object segment irelative to a neighboring object segment j.
 32. A method according toclaim 28, wherein the particular layout parameters of a given pair ofobject segments include the spacing between the object segment i and aneighboring object segment j.
 33. A method according to claim 28,wherein the particular layout parameters of a given one of the objectsegments in the set include the width of the given object segment.
 34. Amethod according to claim 28, wherein the plurality of defect sizesincludes all the defect sizes within a continuous range of defect sizes.35. A method according to claim 28, further comprising the steps of:forming a cost function for post-layout position optimization, the costfunction being dependent upon a group of at least one variable includingthe approximated average critical area; and adjusting at least one ofthe variables to find a minimum of the cost function, wherein the stepof adjusting at least one of the variables includes the steps ofadjusting a layout parameter of at least one of the object segments inthe set and re-calculating the approximated average critical area. 36.Apparatus for approximating the average over a plurality of defect sizesof the short critical area contributed by a set of at least two objectsegments in an integrated circuit fabrication layer, comprising: meansfor summing, over all the object segments i in the set, respectivecritical area contribution values that are dependent upon particularlayout parameters of the objects in the set, each of the contributionvalues being representative of the plurality of defect sizes, and eachof the contribution values being defined such that for each defect sizex in the plurality of defect sizes, and for each pair of mutuallyvisible object segments i and j in the set, for which x exceeds thespacing between object segments i and j, only part of the short criticalarea between object segments i and j is included in the contributionvalue of object segment i and the remainder of the short critical areabetween object segments i and j is included in the contribution value ofobject segment j; and means for approximating the average critical areaas a function of the sum.
 37. Apparatus according to claim 36, whereinin the means for summing, the part of the short critical area betweenobject segments i and j included in the contribution value of objectsegment i is a fraction δ_(x) of the short critical area between objectsegments i and j, 0<=δ_(x)<=1, δ_(x) being constant for all defect sizesx in the plurality of defect sizes.
 38. Apparatus according to claim 37,wherein δ_(x)=0.5.
 39. Apparatus according to claim 36, wherein theplurality of defect sizes includes all the defect sizes within acontinuous range of defect sizes.
 40. Apparatus according to claim 36,further comprising means for forming a cost function for post-layoutposition optimization the cost function being dependent upon a group ofat least one variable including the approximated average critical area;and means for adjusting at least one of the variables to find a minimumof the cost function wherein the means for adjusting at least one of thevariables includes means for adjusting a layout parameter of at leastone of the object segments in the set and re-calculating theapproximated average critical area. 41-42. (canceled)
 43. A method forapproximating the average over a plurality of defect sizes of thecritical area contributed by a set of at least two object segments in anintegrated circuit fabrication layer, comprising the steps of: summing,over all the object segments i in the set, respective critical areacontribution values that are dependent upon particular layout parametersof the objects in the set, each of the contribution values beingrepresentative of the plurality of defect sizes; and approximating theaverage critical area as a function of the sum, wherein the particularlayout parameters include a member of the group consisting of (a) thewidths of the object segments in the set, the widths of first and seconddifferent ones of the object segments in the set being different, and(b) the spacing between pairs of the object segments in the set, thespacing between first and second different pairs of the object segmentsin the set being different.
 44. Apparatus for approximating the averageover a plurality of defect sizes of the critical area contributed by aset of at least two object segments in an integrated circuit fabricationlayer, comprising: means for summing, over all the object segments i inthe set, respective critical area contribution values that are dependentupon particular layout parameters of the objects in the set, each of thecontribution values being representative of the plurality of defectsizes; and means for approximating the average critical area as afunction of the sum, wherein the particular layout parameters include amember of the group consisting of (a) the widths of the object segmentsin the set, the widths of first and second different ones of the objectsegments in the set being different, and (b) the spacing between pairsof the object segments in the set, the spacing between first and seconddifferent pairs of the object segments in the set being different.45-65. (canceled)
 66. A method according to claim 43, wherein theparticular layout parameters include the widths of the object segmentsin the set, the widths of first and second different ones of the objectsegments in the set being different.
 67. A method according to claim 43,wherein the particular layout parameters include the spacing betweenpairs of the object segments in the set, the spacing between first andsecond different pairs of the object segments in the set beingdifferent.
 68. A method according to claim 66, for use in approximatingthe average short critical area over the plurality of defect sizes, andwherein in the step of summing, the contribution values are defined suchthat for each defect size x in the plurality of defect sizes, and foreach pair of visible object segments i and j in the set, for which xexceeds the spacing between object segments i and j, only a fractionδ_(x) of the short critical area between object segments i and j isincluded in the contribution value of object segment i and the remainder(1−δ_(x)) of the short critical area between object segments i and j isincluded in the contribution value of object segment j, 0<=δ_(x)<=1. 69.A method according to claim 68, wherein the fraction δ_(x) is constantfor all defect sizes x in the plurality of defect sizes.
 70. Apparatusaccording to claim 43, wherein the particular layout parameters includethe widths of the object segments in the set, the widths of first andsecond different ones of the object segments in the set being different.71. Apparatus according to claim 43, wherein the particular layoutparameters include the spacing between pairs of the object segments inthe set, the spacing between first and second different pairs of theobject segments in the set being different.
 72. Apparatus according toclaim 44, for use in approximating the average short critical area overthe plurality of defect sizes, wherein in the means for summing, thecontribution values are defined such that for each defect size x in theplurality of defect sizes, and for each pair of mutually visible objectsegments i and j in the set, for which x exceeds the spacing betweenobject segments i and j, only part of the short critical area betweenobject segments i and j is included in the contribution value of objectsegment i and the remainder of the short critical area between objectsegments i and j is included in the contribution value of object segmentj.
 73. Apparatus according to claim 44, for use in approximating theaverage short critical area over the plurality of defect sizes, whereinin the means for summing, the contribution values are defined such thatfor each defect size x in the plurality of defect sizes, and for eachpair of visible object segments i and j in the set, for which x exceedsthe spacing between object segments i and j, one-half of the shortcritical area between object segments i and j is included in thecontribution value of object segment i and one-half of the shortcritical area between object segments i and j is included in thecontribution value of object segment j.